Texturized purification structure incorporating an electrochemical catalyst system

ABSTRACT

A structure for the purification of a polluted gas comprising a porous matrix of an inorganic material, in the form of interconnected grains and an electrochemical system for the treatment of said gas, formed by a reduction catalyst A for reducing the polluting species of the NO x  type, an oxidation catalyst B for oxidizing the polluting species of the hydrocarbon HC type or CO type, an electron-conductive compound C and an ion-conductive compound D, said structure being characterized in that catalysts A and B are placed in the pores of the inorganic material, in that the grains and possibly the grain boundaries of the inorganic material constituting the matrix are covered over at least part of their surface with a texturizing material, and in that said texturizing material is an ionic, electronic or ionic/electronic conductor and constitutes, respectively, the element C, the element D or the elements C and D of the electrochemical gas treatment system.

The present invention relates to the field of structures for purifying a gas laden with gaseous pollutants essentially of the NO type. More particularly, the invention relates to honeycomb structures, especially those used to treat exhaust gases of a gasoline engine or preferably a diesel engine, and incorporating a system that combines a reduction catalyst A for reducing said polluting species of the NO type and an oxidation catalyst B for oxidizing hydrocarbons HC and/or for oxidizing soot and/or steam reforming reactions of the HC+H₂O→3/2H₂+CO type and/or water gas reactions of the CO+H₂O→H₂+CO₂ type.

The techniques and the problems associated with purifying polluted gases, especially at the exit of gasoline or diesel automobile exhaust lines, are well known in the art. A conventional three-way catalyst is used for the combined treatment of NO_(x), CO and HC pollutants and for their conversion into inert and chemically harmless gases such as N₂, CO₂ and H₂O. However, very high efficiency of the system is reached only by continually adjusting the richness of the air/fuel mixture. It is thus known that the slightest deviation from stoichiometry of the mixture causes a substantial increase in pollutant emissions.

To solve this problem, it has been proposed to incorporate with the catalyst materials for temporarily fixing the NO (often called NO_(x) traps in the art) when the mixture is lean (i.e. substoichiometric). However, the major drawback of such a system is that the reduction of the NO, can take place only at the cost of an overconsumption of fuel during a subsequent phase of operation with a rich mixture. The desorption of the trapped NO_(x) on the catalyst and their catalytic reduction to gaseous nitrogen N₂ can be obtained only in the presence, within the reduction catalyst, of a sufficient quantity of reducing species, in the form of hydrocarbons or carbon monoxide CO or else hydrogen H₂, the hydrogen itself possibly being obtained by a catalytic reaction between the hydrocarbons HC and steam or between CO and steam.

At present, no system is known that allows substantial conversion of NO into N₂ in a lean atmosphere, that is to say in the presence of an excess amount of oxygen. One of the aims of the present invention is specifically to provide such a system, in particular enabling a substantial quantity of NO to be converted even when exhaust gases have a lean air/fuel ratio.

An electrochemical catalysis system is well known from U.S. Pat. No. 6,878,354 which makes it possible in theory to convert the NO even when operating outside stoichiometry. The system comprises the combination of an HC and CO oxidation catalyst and an NO reduction catalyst, the two being coupled on the model of an electrochemical cell by means of metallic or inorganic materials. These materials, which provide the junction between the catalysts, are chosen from electron conductors or ion conductors and deliver “continuously”, into the system, the various charged species (for example electrons and oxygen ions O²⁻) necessary for the simultaneous reduction and oxidation reactions. Such systems appear to be advantageous as they enable an electrochemical reaction to take place between a reduction catalyst A and an oxidation catalyst B that are connected together, independently of the conditions of the gaseous medium. According to that publication, such a system makes it possible in particular to increase the catalytic conversion of the polluting species, especially when an engine is operating with a lean mixture.

According to the authors, the presence of such a system comprising an ion conductor D and an electron conductor C allows simultaneous oxidation of the reducing species of the HC, CO, soot and H₂ type and reduction of the oxidizing species of the NO_(x) type.

However, the efficiency of such a system appears limited as its correct operation requires a close contact between the four elements constituting the electrochemical system. Thus, in the embodiments described in the U.S. Pat. No. 6,878,354, catalysts A and B are deposited in the form of particles on the monolith. The efficiency of such a system then depends strongly on the conditions under which catalysts A and B and the electron conductor C and ion conductor D are deposited. This is because the properties obtained are strongly dependent on the dispersion of the various phases corresponding to the various constituents on the support used, a connection being necessary between these four elements in order for the electrochemical system to operate properly.

Furthermore, the efficiency of converting the polluting species may also be substantially limited by the intrinsic characteristics of the materials used as ion and electron conductors. More precisely, since the electrochemical system consists of small particles randomly disposed with respect to one another, its efficiency is necessarily limited, on the one hand, by the connections between the particles, and, on the other hand, by the small quantity of conducting species (electrons and/or ions) that are available for the electrochemical catalyst system to operate properly. Most particularly, the procedures for depositing catalysts A and B in the pores of the support, especially the process conditions for impregnation with the solutions comprising the catalyst, appear to be very difficult to determine, experience showing that an ideal distribution of the active sites promoting successive contacts of the ADB or BDA or ACB or BCA type, necessary for the optimum operation of the electrochemical system, is very difficult to implement and to reproduce. In particular, problems arise in the distribution of the active sites in cases in which the impregnation deposition process leads to deposited thicknesses that are locally too high or irregular.

Furthermore, problems of adhesion of the catalyst A or B impregnation solution to the porous substrate also appeared. The lack of adhesion is characterized in particular by a final coating that does not have the homogeneity and uniformity characteristics required for optimum operation of the electrochemical system. This is all the more critical on the matrices in the form of interconnected grains, the surface of which is relatively smooth and/or convex, especially matrices obtained by interconnection between silicon carbide (SiC) microcrystallites.

One possible solution for solving the above deposition problems could be to increase the concentration of catalyst A and B particles in the solutions used for the impregnation. However, increasing the quantity of material then deposited in the pores of the inorganic material results in a substantial increase in the pressure drop associated with the filtering structure, this being highly prejudicial to the application as a particulate filter. It should be clearly noted that any compensation by an increase in the porosity of the filtering structure would then necessarily lead to a drop in the mechanical and thermomechanical strength of the porous structure.

In addition to the need to solve the above-mentioned implementation problems, there is also a need to obtain a structure having a catalytic performance that is sufficiently stable over time. More particularly, the catalytic activity must remain acceptable throughout the lifetime of the filter, according to the current and future pollution control standards.

The trials carried out by the applicant have also shown that an aging problem arises in the case of the catalytic system described in U.S. Pat. No. 6,878,354. This problem can be partly solved by a thicker coating. However, such a coating would lead to the same problems mentioned above and would require the use of a larger amount of noble metals within the catalyst.

The object of the structure according to the invention is to solve the abovementioned problems by providing an electrocatalyzed support that is particularly suitable for filtering applications and has an improved purification performance, especially in respect of the amount of NO_(x) reduced, especially when the engine is operating in lean mixture mode, the pressure drop and the aging resistance.

More particularly, the present invention relates to a structure, preferably a honeycomb structure, for the purification of a polluted gas, for example an exhaust gas of a diesel or gasoline engine, comprising:

-   -   a porous matrix of an inorganic material, in the form of grains         that are interconnected so as to provide cavities between them,         such that the open porosity of which is between 20 and 70% and         the median diameter of its pore distribution is between 5 and 40         μm; and     -   an electrochemical system for the treatment of said gas, formed         by:         -   a. a reduction catalyst A for reducing the polluting species             of the NO_(x) type,         -   b. an oxidation catalyst B for oxidizing hydrocarbons HC,         -   c. an electron-conductive compound C and         -   d. an ion-conductive compound D,             said catalysts A and B being in electronic contact via             compound C and in ionic contact via compound D, said             structure being characterized:     -   in that catalysts A and B are placed in the pores of the         inorganic material;     -   in that the grains and possibly the grain boundaries of the         inorganic material are covered over at least part of their         surface with a texturizing material, said texturizing consisting         of irregularities having dimensions between 10 nm and 5 microns;         and     -   in that said texturizing material is an ionic, electronic or         ionic/electronic conductor and constitutes, respectively, the         element C, the element D or the elements C and D of the         electrochemical gas treatment system.

For example, said irregularities take for example the form of beads, crystallites, polycrystalline clusters, or even rods or acicular structures, hollows or craters, said irregularities having a mean diameter d of between about 10 nm and about 5 microns and a mean height h or a mean depth p of between about 10 nm and about 5 microns.

The term “mean diameter d” is understood within the meaning of the present description to be the mean diameter of the irregularities, these being individually defined from the plane tangential to the surface of the grain or of the grain boundary on which they are located.

The term “mean height h” is understood within the meaning of the present description to be the mean distance between the top of the relief formed by the texturizing and the aforementioned plane.

The term “mean depth p” is understood within the meaning of the present description to be the mean distance between, on the one hand, the deepest point formed by the impression, for example the hollow or crater of the texturizing, and, on the other hand, the aforementioned plane.

According to one possible embodiment, the mean diameter d of the irregularities is between 100 nm and 2.5 microns.

For example, the mean height h or the mean depth p of the irregularities is between 100 nm and 2.5 microns.

According to a preferred embodiment, the texturizing material covers at least 10% of the total surface of the grains and possibly of the grain boundaries of the inorganic material constituting the porous matrix. Preferably, the texturizing material covers at least 15% of the total surface of the grains and possibly of the grain boundaries of the inorganic material constituting the porous matrix.

Typically, the mean equivalent diameter d and/or the mean height h or the mean depth p of the irregularities are/is smaller than the mean size of the grains of the inorganic material constituting the matrix by a factor of between ½ and 1/1000.

For example, the mean equivalent diameter d and/or the mean height h or the mean depth p of the irregularities are/is smaller than the mean size of the grains of the inorganic material constituting the matrix by a factor of between ⅕ and 1/100.

According to one possible embodiment, the texturizing material is of the same nature as the inorganic material constituting the matrix.

The expression “of the same nature” is understood, within the context of the present invention, to mean that the texturizing material and the inorganic material constituting the matrix are based on one and the same compound, for example SiC, that is to say that said compound (e.g. SiC) is present in an amount of at least 25% by weight in both materials, preferably at least 45% by weight in both materials and very preferably at least 70% by weight in both materials.

The inorganic material constituting the matrix is for example based on silicon carbide SiC. In particular, the inorganic material may be based on doped SiC, for example doped with aluminum or with nitrogen, in such a way that its electronic resistivity is less than 20 Ω·cm at 400° C.

According to a first embodiment, the irregularities are formed by crystallites or by a cluster of crystallites of a fired or sintered material on the surface of the grains of the porous matrix.

According to another embodiment, the irregularities essentially consist of beads of an electron-conductive and/or ion-conductive material.

Alternatively, the irregularities may also take the form of craters hollowed out in a fired or sintered material on the surface of the grains of the porous matrix.

The invention also relates to the intermediate structure for obtaining a catalytic filter for the treatment of solid particles and gaseous pollutants according to one of the above embodiments and comprising a porous matrix consisting of an inorganic material, in the form of grains that are interconnected so as to provide cavities between them, such that the open porosity is between 20 and 70% and the median pore diameter is between 5 and 40 μm, said grains of the inorganic material being covered over at least part of their surface with a texturizing material according to one of the preceding claims.

The invention also relates to a process for obtaining a filter as described above and comprising the following steps:

-   -   forming and firing of a honeycomb structure consisting of a         porous matrix of an inorganic material, in the form of grains         that are interconnected so as to provide cavities between them,         such that the open porosity is between 30 and 60% and the median         pore diameter is between 5 and 40 μm;     -   deposition, on the surface of at least some of the grains of the         honeycomb structure, of a texturizing material having for         example the form of beads, crystallites, polycrystalline         clusters, hollows or craters; and     -   successive impregnation of the textured honeycomb structure with         one or more solutions comprising catalysts A and B or a catalyst         precursor and optionally the electron-conductive and         ion-conductive materials or their precursors.

According to the process, the texturizing material is deposited by the application of a slip of said material for covering the surface of the grains, followed by a firing or sintering heat treatment, by the application of a sol-gel solution that includes a filler in the form of inorganic beads or particles, followed by a firing or sintering heat treatment or else by the application of a sol-gel solution that includes a filler in the form of organic beads or particles, followed by a firing or sintering heat treatment.

More precisely, the texturizing process according to the invention may be obtained either:

-   -   1) by deposition of a suspension, such as for example a slip         consisting of a powder and a powder mixture preferably in a         liquid such as water, or a sol-gel filled with mineral         particles, or an organic or organo-mineral sol-gel, leading         after a heat treatment to a material of crystalline and/or         glassy inorganic nature, preferably of ceramic and with a         thermal stability substantially equal to or even greater than         that of alumina, which is most often the principal constituent         of the catalytic coatings of the prior art. The deposition is         followed by one or more heat treatments of the substrate,         preferably in air but possibly in a controlled atmosphere, for         example in argon or nitrogen, if this is necessary in particular         to prevent deterioration or oxidation of the substrate or of the         coating for example. It may also be envisioned to carry out this         texturizing on the green or partially fired substrate provided         that the mechanical strength and integrity of the substrate are         sufficient for the texturizing operation to be carried out and         provided that the firing conditions enable the aforementioned         texturizing characteristics to be obtained. In the case of         suspensions, in addition to the powder(s) of inorganic         (preferably ceramic) nature or their precursors, for example in         the form of an organo-metallic compound, the formulation may         contain additions taken from the following list: one or more         dispersants (for example, an acrylic resin or an amine         derivative); a binder of organic nature (for example an acrylic         resin or a cellulose derivative) or even of mineral nature (for         example clay); a wetting or film-forming agent (for example, a         polyvinyl alcohol PVA); and one or more pore formers (for         example polymers, latices, polymethyl methacrylate), some of         these components possibly combining several of these functions.         Just like the form and the particle size of the powders or         precursors and the nature of the suspension liquid, the nature         and the amount of these additions have an impact on the size of         the microtexturizing and its location on the substrate. The         preferred texturizing must be carried out on the surface of the         grains but also partly on the grain boundaries;     -   2) or by starting from a powder or a powder mixture via a         carrier gas. Direct deposition starting from liquid or gaseous         species, for example by PVD (physical vapor deposition) or CVD         (chemical vapor deposition), is also possible.

Other texturizing methods may also be employed according to the invention, such as heat treatment in a gas (for example O₂ or N₂ in the case of a substrate based on SiC). Plasma etching or chemical etching processes may also be used to obtain the texturizing according to the invention, depending on the operating conditions and on the nature of the substrate.

The catalytic coating according to the invention is typically obtained by impregnation with one or more successive solutions comprising the catalysts of the electrochemical system according to the invention in the form of the support material or its precursors and of an active phase or a precursor of the active phase. In general, the precursors used take the form of organic or mineral salts or compounds, dissolved or in suspension in an aqueous or organic solution. The impregnation is followed by a heat treatment for the purpose of obtaining the final coating of a solid and catalytically active phase in the pores of the filter.

Such processes, and the devices for implementing them, are for example described in the patent applications or patents US 2003/044520, WO 2004/091786, U.S. Pat. No. 6,149,973, U.S. Pat. No. 6,627,257, U.S. Pat. No. 6,478,874, U.S. Pat. No. 5,866,210, U.S. Pat. No. 4,609,563, U.S. Pat. No. 4,550,034, U.S. Pat. No. 6,599,570, U.S. Pat. No. 4,208,454 or U.S. Pat. No. 5,422,138.

Catalyst A used for the reduction reaction is chosen from the catalysts that are well known in the art for their activity and preferably for their selectivity with respect to NO_(x) reduction reactions. They may in particular be chosen from compounds of the alkali metal or alkaline earth or rare earth type, which also act as NO_(x) traps, for example such as those described in application EP 1 566 214, in that they are deposited as a mixture with an active principle that includes precious metals (Pt, Pd, Rh) by adsorption on the surface of a powder having a high specific surface area, for example alumina powder.

Catalyst B used for the hydrocarbon oxidation reaction is chosen from the catalysts well known in the art for their activity and preferably their selectivity with respect to hydrocarbon oxidation reactions. In particular, reforming and steam reforming catalysts used in the petrochemical and refining field may be used according to the invention.

The arrangement according to the invention has, compared with the nontexturized structures known hitherto, many advantages, among which:

-   -   the introduction of the catalytic system in the pores of the         support advantageously makes it possible to greatly increase the         developed surface area of catalysts accessible to the         pollutants, and consequently the probability of contact and         exchange between the reactive species;     -   the introduction of a microtexturizing on the surface of the         grains constituting the matrix makes it possible to further         increase this surface area;     -   according to the invention, the support constitutes either the         electron conductor C, or the ion conductor D, or the         ion/electron conductors C and D. Advantageously, this         arrangement makes it possible to supply the electrochemical         system with an unlimited quantity of charged species (ions         and/or electrons), thus appreciably improving the capacity of         the system;     -   a limited number of constituents of the system must be deposited         on the support, thereby greatly reducing the dependence of the         system performance relative to the catalyst deposition         conditions on the support;     -   good chemical compatibility between the porous inorganic         material constituting the support and the catalytic system;     -   reduction in production costs owing to a simpler deposition         method because of the lesser number of compounds to be         deposited;     -   an increase in the catalytic efficiency, a larger quantity of         catalyst being able to be deposited because of the limited         number of constituents that have to be deposited in the pores of         the matrix, without corresponding increase in the pressure drop;         and     -   an appreciably extended lifetime of the catalytic activity of         the texturized filter.

For example, the porous inorganic material comprises or is formed by an electron-conductive inorganic material of the carbide type, for example SiC, or of the silicide type, for example MoSi₂, or a boride, for example TiB₂, or of the La_(1-x)Sr_(x)MnO₃ family or of the mixed cerium gadolinium oxide (CGO) type.

The porous inorganic material may also comprise or be formed by an inorganic material that conducts by oxygen ions, of the fluorite structure, for example zirconia stabilized by CaO or by Y₂O₃, or mixed cerium gadolinium oxides, or of perovskite structure, for example a gallate, compounds based on lanthanum of the LaAlO₃ or LaGaO₃ or La_(1-x)Sr_(x)Ga_(1-y)Mg_(y)O₃ type or of the BIMEVOX structure, for example Bi₂V_(1-x)Me_(x)O_(z) or of the LAMOX structure, for example La₂Mo₂O₉, or else of the apatite structure, for example Me₁₀(XO₄)₆Y₂, or of the mixed cerium gadolinium oxide (CGO) type. CGOs have the advantage of being both ion conductors and electron conductors.

The porous inorganic material may comprise or be formed by an inorganic proton-conductive material of the perovskite type, for example SrCe_(1-x)M_(x)O_(3-α) where M is a rare earth, typically the compound SrCe_(x)Yb_(1-x)O_(3-α), or of the BaCe_(1-x)M_(x)O_(3-α) type, for example the compound BaCeO₃, or else a compound of the La_(x)Sr_(1-x)ScO_(3-α) family, for example La_(0.9)Sr_(0.1)ScO_(3-α).

According to a preferred embodiment, the porous inorganic material is based on silicon carbide SiC, preferably recrystallized at a temperature between 2100 and 2400° C. In particular, the inorganic material may be based on doped SiC, for example doped with aluminum or with nitrogen, and in such a way that its electronic resistivity is preferably less than 20 Ω·cm, more preferably less than 15 Ω·cm and even more preferably less than 10 Ω·cm at 400° C. The expression “based on silicon carbide” is understood, within the context of the present description, to mean that the material consists of at least 25%, preferably at least 45% and very preferably at least 70% by weight of SiC.

The porous inorganic material may also comprise or be formed by a mixture of optionally doped silicon carbide and at least one inorganic material conducted by oxygen ions, for example of the fluorite structure (for example, zirconia stabilized by CaO or by Y₂O₃, mixed cerium gadolinium oxides), or of perovskite structure (a gallate, or compounds based on lanthanum, for example LaAlO₃ or LaGaO₃ or La_(1-x)Sr_(x)Ga_(1-x)Mg_(y)O₃), or of the BIMEVOX structure (for example Bi₂V_(1-x)Me_(x)O_(z)), or of the LAMOX structure (for example La₂Mo₂O₉), or of apatite structure (for example Me₁₀(XO₄)₆Y₂).

According to another embodiment, the porous inorganic material comprises or is formed by a mixture of optionally doped silicon carbide and at least one inorganic proton-conductive material, for example of the perovskite type (for example SrCe_(1-x)M_(x)O_(3-α), where M is a rare earth, for example the compound SrCe_(x)Yb_(1-x)O_(3-α)) or of the BaCe_(1-x)M_(x)O_(3-α) type (for example the compound BaCeO₃), or else a compound of the La_(x)Sr_(1-x)ScO_(3-α) family (for example La_(0.9)Sr_(0.1)ScO_(3-α)).

In another possible embodiment, the porous inorganic material comprises or is formed by optionally doped silicon carbide, in the pores of which a mixture of reduction catalyst A, of oxidation catalyst B and of at least one inorganic oxygen-ion-conductive material D, for example of fluorite structure (such as zirconia stabilized by CaO or Y₂O₃ or mixed cerium gadolinium oxides), or of perovskite structure (gallate, lanthanum-based compounds of the LaAlO₃ or LaGaO₃ or La_(1-x)Sr_(x)Ga_(1-y)Mg_(y)O₃ type), or of BIMEVOX structure (for example Bi₂V_(1-x)Me_(x)O_(z)), or of LAMOX structure (for example La₂Mo₂O₉) or of apatite structure (for example Me₁₀(XO₄)₆Y₂), is deposited.

According to another embodiment, the porous inorganic material comprises or is formed by optionally doped silicon carbide in the pores of which a mixture of reduction catalyst A, of oxidation catalyst B and of at least one inorganic proton-conductive material D, for example of the perovskite type (for example SrCe_(1-x)M_(x)O_(3-α) where M is a rare earth, for example the compound SrCe_(x)Yb_(1-x)O_(3-α)) or of the BaCe_(1-x)M_(x)O_(3-α) type (for example the compound BaCeO₃), or else a compound of the La_(x)Sr_(1-x)ScO₃, family (for example La_(0.9)Sr_(0.1)ScO_(3-α)), is deposited.

The present invention is most particularly applicable in the structures used for the purification and filtration of a diesel engine exhaust gas. Such structures, generally referred to as particulate filters, comprise at least one and preferably a plurality of honeycomb monoliths. Unlike the purification devices described above, in such filters, said monolith or monoliths comprise a plurality of adjacent ducts or channels having mutually parallel axes separated by porous walls, said ducts or channels being closed off by plugs at one or other of their ends in order to define inlet ducts opening onto a gas intake face and outlet ducts opening onto a gas discharge face, in such a way that the gas flows through the porous walls. Examples of such assembled or non-assembled structures are for instance described in the publications EP 0 816 065, EP 1 142 619, EP 1 306 358 or EP 1 591 430.

In such filtering structures, the gases are forced to flow through the walls. The work carried out by the applicant has shown that the use of an electrochemical catalyst system on the texturized matrix as described above makes it possible, surprisingly, to achieve a very high conversion of the polluting species without a corresponding increase in the pressure drop caused by introducing the filter into the exhaust line.

Such a system also helps to improve the efficiency in regenerating the filter by promoting a higher rate of soot oxidation.

The invention and its advantages will be better understood upon reading the following nonlimiting examples of the present invention, which are provided merely by way of illustration.

EXAMPLE 1 Comparative Example

An assembled ceramic filter made of SiC, the open porosity of the filtering walls of which is close to about 40%, was firstly synthesized using well-known techniques. The synthesis was carried out under conditions enabling the dopant Al to be incorporated in a proportion by mass of about 200 ppm. Such doping provides a structure having a substantially improved electronic conductivity, i.e. a resistivity of less than 10 Ω·cm at 400° C.

More precisely, the filtering structure is obtained by assembling silicon carbide filtering elements that were firstly extruded, dried, and then fired, using well-known techniques, and bonded together using a jointing cement according to the techniques described for example in patent EP 1 142 619. The filtering parts were characterized by a plurality of adjacent ducts or channels having mutually parallel axes separated by porous walls, said ducts or channels being closed off by plugs at one or other of their ends in order to define inlet ducts opening onto a gas intake face and outlet ducts opening onto a gas discharge face, in such a way that the gas passes through the porous walls.

In this example, two fractions of silicon carbide grains having an Al mass content of about 200 ppm were initially used. A first fraction had a median diameter d₅₀ of between 5 μm and 50 μm, at least 10% by weight of the grains making up this fraction having a diameter greater than 5 μm. The second fraction had a median grain diameter of less than 5 μm. The two fractions were mixed in a mass ratio of 1 with a temporary binder of the methyl cellulose type and a polyethylene organic pore-forming agent.

The main geometric characteristics of the filter thus obtained are given in Table 1.

TABLE 1 Channel geometry Square Channel density 180 cpsi (channels per square inch, 1 inch = 2.54 cm, i.e. about 28 channels/cm²) Wall thickness 350 μm Length 15.2 cm Width 3.6 cm Volume 2.47 liters Porosity About 47% Mean pore diameter About 15 μm

Catalysts A and B and the ion-conductive compound D were synthesized in the following manner.

Catalyst A:

500 g of a gamma-alumina powder sold by Sasol were impregnated with an aqueous barium nitrate solution. Next, the whole was dried at 110° C. and then calcined at 600° C. for 3 hours in air, so as to obtain a powder consisting of BaO-coated alumina grains. This powder was then impregnated using well-known techniques, in an aqueous platinum dinitrodiamine chloride solution, then dried at 110° C. for 3 hours before finally being heated at 250° C. for 2 hours so as to obtain catalyst A.

Catalyst B:

300 grams of a zeolite powder of mordenite type were suspended in a zirconium hydroxynitrate solution, to which an aqueous ammonium solution was added so as to adjust to at least pH 8. Next, the solution was filtered, dried at 110° C. and then calcined at 500° C. for 1 hour. The powder thus obtained was dispersed in an aqueous rhodium nitrate solution, then filtered and dried at 400° C. for 1 hour in order to obtain catalyst B.

Ion Conductor D:

The ion conductor D used was a YSZ powder (zirconia powder, basic grade TZ), sold by Tosoh.

Particle size of the powders of catalysts A, B and ion conductor D was adapted and chosen according to the porosity of the porous ceramic body. The median diameter of these powders was in particular chosen to be less than 5 μm.

In a second step, the as-formed filter structure was then immersed in a bath of an aqueous solution containing catalysts A, B and compound D, in proportions making it possible to obtain about 2% by weight, relative to the total weight of the support, of each compound on the SiC support.

The filter was impregnated with the solution according to a method of implementation similar to that described in U.S. Pat. No. 5,866,210. Next, the filter was dried at about 150° C. and then heated at a temperature of about 500° C. A reference electrocatalytic filter was thus obtained.

EXAMPLE 2 According to the Invention

The as-formed structure obtained according to Example 1 was subjected to a first texturizing treatment before the incorporation of catalysts A and B and of ion conductor D.

During this treatment, a texturizing material was introduced into the pores of the filter in the form of a slip. More precisely, a suspension based on SiC doped with about 200 ppm of aluminum was prepared.

The suspension comprised, in percent by weight, 96% water, 0.1% of a nonionic dispersant, 1% of a PVA (polyvinyl alcohol) binder and 2.8% of an SiC powder of 0.5 μm median diameter, the purity of said powder being greater than 98% by weight.

Such a doping level makes it possible, according to a first advantage, to obtain a structure having a substantially improved surface electronic conductivity, i.e. a resistivity of less than 10 Ω·cm at 400° C. The texturized structure thus exhibited surface electronic conductivity and thus constituted element C of the system.

The slip or suspension was prepared according to the following steps:

The PVA, used as binder, was firstly dissolved in water heated to 80° C. The dispersant was introduced into a tank, kept stirred and containing the PVA dissolved in water, before being followed by the SiC powder until a homogeneous suspension was obtained.

The slip was deposited in the filter by simple immersion, the excess suspension being removed by vacuum suction, under a residual pressure of 10 mbar. The filter thus obtained underwent a drying step at 120° C. for 16 hours followed by a sintering heat treatment at 1700° C. for 3 h in argon.

FIG. 2 shows an SEM photograph, in cross section, of the filtering walls of the texturized filter thus obtained, showing the irregularities on the surface of the SiC grains constituting the porous matrix. The irregularities take the form of SiC crystallites or clusters of crystallites. This FIG. 2 should be compared with FIG. 1, which corresponds to the as-formed structure consisting of the SiC grains before the texturizing treatment.

According to this embodiment, the measured parameter d corresponds to the mean diameter, within the meaning described above, of the crystallites present on the surface of the SiC grains, i.e. about 0.5 μm. The parameter h corresponds to the mean height h of said crystallites, i.e. about 0.5 μm. This coating covers about 18% of the total offered surface area of the SiC grains.

Secondly, the as-formed filter structure was then immersed in a bath of an aqueous solution containing catalysts A and B and compound D, in the same proportions and under the same principles and operating method as Example 1. In particular, the filter was impregnated, dried and heated using the same operating method as in the case of Example 1.

An electrocatalytic filter according to the invention was thus obtained.

Thirdly, the structure was again dried at 150° C. then heated at a temperature of about 500° C. in air so as to obtain a structure according to the invention.

The properties of the catalytic filters obtained according to Examples 1 and 2 were evaluated using various tests.

1) NO_(x) Conversion Test:

The performance of the filter was measured at a temperature of 400° C. using two synthetic gas mixtures according to Table 2, these being characteristic of the exhaust gases for a diesel engine operating with a lean mixture (mixture 1) and for a diesel engine operating with a rich mixture (mixture 2).

TABLE 2 Constituent Mixture 1 (lean) Mixture 2 (rich) HC (ppmv) 1000 1000 CO (ppmv) 600 600 NO_(x) (ppmv) 500 500 CO₂ (vol %) 6 6 H₂O (vol %) 10 10 O₂ (vol %) 10 0.5 N₂ balance balance

The test was carried out in the following manner: the lean gas mixture 1 firstly flowed over the catalyzed filter maintained in an electric furnace at 400° C. Every two minutes, the gas composition was switched onto the rich gas mixture 2 for 5 seconds, before being switched back to the mixture 1, and so on. The composition of the gases leaving the furnace was analyzed after stabilization so as to determine the amount of NO_(x) converted.

The test as described above was carried out under the same conditions on the electrocatalyzed filter according to Example 1 (non texturized filter) and on the electrocatalyzed filter according to Example 2 (texturized filter).

TABLE 3 Filter according Filter according to Example 1 to Example 2 Degree of NO_(x) 50 55 conversion (vol %)

The results given in Table 3 show that the filter according to the invention (Example 2) has a higher degree of NO_(x) conversion than the comparative filter (Example 1).

2) Pressure Drop Test:

The pressure drop was measured on the filter according to the prior art, for an air flow rate of 600 m³/h in a stream of ambient air. The term “pressure drop” is understood, within the context of the present invention, to mean the pressure difference that exists between the upstream side and the downstream side of the filter.

The experimental results are given in Table 4.

TABLE 4 Filter according Filter according to Example 1 to Example 2 Pressure drop 53 53 measured in mbar

Surprisingly, it may be seen, by comparing the data in Table 4, that the pressure drop of the filter according to the invention is not increased despite the microtexturizing and is similar to that on the reference filter.

3) Aging Test:

To estimate the aging resistance capacity of the filter obtained according to Example 1 and the filter obtained according to Example 2, they were subjected to accelerated aging, compared with the normal operating conditions in an exhaust line.

The filters were placed in a furnace at 800° C. in a wet air atmosphere for a period of 5 hours, such that the molar concentration of water was kept constant at 3%.

The degree of NO_(x) conversion on the filters thus aged was measured using the same experimental protocol as previously (see test 1).

TABLE 5 Aged filter Aged filter according to according to Example 1 Example 2 Degree of NO_(x) 49 54 conversion (vol %)

After aging, the results indicated in Table 5 show that the filter according to the invention (Example 2) still has a higher degree of NO_(x) conversion than the comparative filter (Example 1). 

1. A structure for the purification of a polluted gas selected from the group consisting of an exhaust gas of a diesel or gasoline engine, comprising: a porous matrix of an inorganic material, in the form of grains that are interconnected so as to provide cavities between them, the open porosity of which is between 20 and 70% and the median diameter of its pore distribution is between 5 and 40 μm; and an electrochemical system for the treatment of said gas comprising: a. a reduction catalyst A for reducing a polluting species of the NO_(x) type, b. an oxidation catalyst B for oxidizing a polluting species of the hydrocarbon HC type or CO type, c. an electron-conductive compound C and d. an ion-conductive compound D, said catalysts A and B being in electronic contact via compound C and in ionic contact via compound D, wherein: catalysts A and B are placed in the pores of the inorganic material; the grains and possibly the grain boundaries of the inorganic material constituting the matrix are covered over at least part of their surface with a texturizing material, said texturizing consisting of irregularities having dimensions between 10 nm and 5 microns; and said texturizing material is an ionic, electronic or ionic/electronic conductor and constitutes, respectively, the element C, the element D or the elements C and D of the electrochemical gas treatment system.
 2. The structure as claimed in claim 1, in which said texturizing consists of irregularities taking the form of beads, crystallites, polycrystalline clusters, rods or acicular structures, hollows or craters, said irregularities having a mean diameter d of between about 10 nm and about 5 microns and a mean height h or a mean depth p of between about 10 nm and about 5 microns.
 3. The structure as claimed in claim 1, in which the mean equivalent diameter d and/or the mean height h or the mean depth p of the irregularities are/is smaller than the mean size of the grains of the inorganic material constituting the matrix by a factor of between ½ and 1/1000.
 4. The structure as claimed in claim 1, in which the texturizing material covers at least 10% of the total surface of the grains and possibly of the grain boundaries of the inorganic material constituting the porous matrix.
 5. The structure as claimed in claim 1, in which the texturizing material is of the same nature as the inorganic material constituting the matrix.
 6. The structure as claimed in claim 1, in which the inorganic material constituting the matrix is based on silicon carbide, SiC.
 7. The structure as claimed in claim 6, in which the inorganic material is based on SiC doped with aluminum or with nitrogen, and is doped in such a way that its electronic resistivity is less than 20 Ω·cm at 400° C.
 8. The structure as claimed in claim 1, in which the irregularities are formed by crystallites or by a cluster of crystallites of a fired or sintered material on the surface of the grains of the porous matrix.
 9. The structure as claimed in claim 1, in which the irregularities essentially consist of beads of an electron-conductive and/or ion-conductive material.
 10. The structure as claimed in claim 1, in which the irregularities take the form of craters hollowed out in a fired or sintered material on the surface of the grains of the porous matrix.
 11. The structure as claimed in claim 1 for the purification and filtration of an exhaust gas of a diesel engine, comprising at least one honeycomb monolith, said monolith(s) comprising a plurality of adjacent ducts or channels having mutually parallel axes separated by porous walls, said ducts or channels being closed off by plugs at one or other of their ends in order to define inlet ducts opening onto a gas intake face and outlet ducts opening onto a gas discharge face, in such a way that the gas passes through the porous walls. 